Method for coating the inner surface of a heat exchanger with a powdered solid catalyst

Abstract

The present invention relates to a method for depositing a coating on the internal surfaces forming the cavity of a heat exchanger, characterized in that the coating comprises a liquid adhesive and a powder of a powdered solid intended to act as a catalyst for a physicochemical reaction.

Description

[Technical field]

[0001] Designation of related technical field The present invention relates to the manufacture of coatings which act as catalysts for physico-chemical reactions on the metallic internal surfaces of brazed plate heat exchangers, particularly low-temperature heat exchangers.

[0002] The technical problem that the invention addresses A brazed plate-and-wave heat exchanger is a heat exchanger typically formed by a metal assembly of brazed plates and corrugations. This type of heat exchanger produces a very compact unit with a large heat exchange surface area and low pressure drop. This type of heat exchanger is composed of a set of parallel plates between which intermediate elements, specifically corrugated or wave-like structures, are inserted. These corrugated structures form cavities with metallic inner surfaces. The stacked plates form between them a stack of flat passages for the different fluids to be brought into exchange relationship.

[0003] Such heat exchangers are characterized by metallic inner surfaces that form at least one fluid-flowing cavity, the characteristic size of which is on the order of millimeters and has a high aspect ratio (length / width ratio).

[0004] The plates and spacers in this type of heat exchanger are typically made of metal.

[0005] There are many techniques available for coating metal substrates, but most of them do not work for coating the interior walls of the metal cavities in such heat exchangers.

[0006] Chemical vapor deposition (CVD) uses gaseous precursors of the coating to be produced. This precursor can be produced in direct proximity to the surface to be coated (pack cementation) or transported via gas onto the surface to be coated (ex-pack). When transported via gas, the main limitation of this technique is the rapid depletion of the gas mixture in reactive species, resulting in non-uniformity in chemical composition and / or thickness, or even a lack of deposition on the surfaces furthest from the carrier gas supply in the case of large structures.

[0007] An improvement of this method consists in using a cement consisting of a powder of the metal to be deposited, an inert diluent and an additive which may be a stripping flux, as described in EP 2956566. The viscosity of the cement is adjusted to improve its flowability and to allow the filling of the cavities to be coated. In another variation taught by EP 3049545, the deposition is carried out by liquid means using an aqueous suspension containing a powder of the metal to be deposited and at least one additive which may be selected from binders, dispersants, etc., the purpose of which is to promote the wetting of the surface to be coated.

[0008] A heat-activated consolidation step is then required to extract the organic phase (debinding) and densify the coating, which does not necessarily have zero effect on the mechanical properties of the coated structure, especially if the process requires high temperatures.

[0009] This problem is even more severe when the material constituting the deposited coating is highly reactive with respect to the substrate, for example when the material of the deposited coating is based on iron oxide and the substrate is based on aluminum, the problem arising from the combination of these two materials being highly exothermic. In this example, maintaining iron oxide deposits on the surface of the aluminum-based substrate during high-temperature assembly operations is not easy, especially when the assembly is carried out by brazing.

[0010] This difficulty can be alleviated by installing a diffusion barrier to isolate the coating from the substrate during consolidation, an approach that significantly complicates the manufacturing process.

[0011] Another problem with such coatings is the formation of deposits whose growth partially damages the substrate, especially when the consolidation heat treatment leads to the formation of intermetallic compounds between the substrate and the coating.

[0012] This is one of the major drawbacks with the regulations governing pressure vessels, whose mechanical strength is defined based on the minimum substrate thickness that can take the form of a wall, especially when the intermetallic compounds formed between the substrate and the cladding are inherently brittle, such as those in the Fe-Al system.

[0013] For conventional devices, this can be easily avoided by adding wall thickness, but this cannot be assumed for brazed plate heat exchangers, whose performance depends in part on the thickness of the walls that make up the plate and corrugated structure. In fact, in this type of heat exchanger, the wall thinning allowed is comparable to or less than the wall thickness consumed during the coating consolidation stage. This requires a perfect control of the precursor deposition conditions, since the residual wall thickness cannot be controlled after the fact.

[0014] Another drawback of these techniques is the need to produce a generally smooth coating without irregularities that would increase the reactive surface of the deposit.

[0015] To the best of the inventors' knowledge, conventional coating techniques are unable to achieve an adherent deposit of iron oxide on the surfaces of aluminum cavities having small cross sections and large length-to-width ratios of greater than 1000.

[0016] Whether deposition is performed prior to assembly by PVD or CVD, or after assembly using slips, the subsequent substrate heating required for consolidation or assembly of the coating results in the formation of intermetallic compounds that are detrimental to the mechanical integrity of the structure, especially since these intermetallic compounds (Al-Fe) are known to be sensitive to hydrogen embrittlement.

[0017] The present invention provides a novel solution to these problems. Summary of the Invention

[0018] According to a first aspect of the invention, a method is proposed for producing a coating on the metallic internal surfaces forming the cavities of a heat exchanger, the coating intended to act as a catalyst for the physicochemical reaction of the ortho-para conversion of hydrogen, the method comprising the following steps: - depositing a liquid adhesive onto an internal metal surface, the liquid adhesive being water-based, organic or inorganic; and depositing a powder of a powdered solid on the deposited liquid adhesive, the powdered solid being a mineral and the powder having a particle size of 10 μm to 100 μm.

[0019] The heat exchanger may be a low temperature exchanger.

[0020] The coating (liner) obtained on the metallic internal surface of the exchanger according to the present method thus acts as a catalyst for the physicochemical reactions that take place in the exchanger during its operation. The powdered mineral solid is the catalyst. In this way, the heat exchanger combines the function of a conventional heat exchanger, which transfers heat between fluids, with that of a physicochemical reactor.

[0021] The liquid adhesive enables the powdered solids to adhere to the internal metallic surfaces of the exchanger, i.e., adhere the mineral phase to the metal surface.

[0022] According to one embodiment of the invention, the method includes, after the powder deposition step, a subsequent step of maintaining the inner metal surface at ambient temperature, during which the solvent of the liquid adhesive is at least partially removed by evaporation.

[0023] Depending on the nature of the adhesive, for example in the case of thermosetting adhesives, holding the adhesive at ambient temperature may be sufficient for the adhesive to dry and acquire all its adhesive properties. The holding time is selected to evaporate all the solvent from the adhesive, or the desired proportion of the solvent. In particular, it depends on the geometric characteristics of the exchanger and the nature of the adhesive. For example, it may take from a few minutes to 2 hours at temperatures between ambient and 100 °C.

[0024] According to another embodiment of the invention, the method according to the invention comprises, after the powder deposition step, a subsequent step of polymerizing the liquid adhesive by heat treatment carried out in a temperature range that does not allow the metallic inner surface to interact with the powder solids. Depending on the nature of the adhesive, for example in the case of organic binders, polymerization may be necessary in order to dry and obtain its full adhesive properties. The adhesive is selected so that polymerization can occur in a sufficiently low temperature range so that the metallic inner surface of the exchanger does not interact with the powder solids. At higher temperatures, close to the melting point of the brazing alloy used in aluminum brazed plate heat exchangers, the aluminum and the brazing alloy become very reactive with oxide particles or mixtures of oxides, forming powder solids. This has the effect of forming intermetallic compounds between the substrate and the coating and reducing the thickness of the aluminum wall.

[0025] Thus, the liquid adhesive used to attach the powdered solid particles to the metallic internal surface of the exchanger is, depending on its nature, either removed by evaporation or polymerized during heat treatment.

[0026] According to one example, the heat treatment is carried out at a temperature below 200° C. By limiting the temperature at which the polymerization is carried out to 200° C., any risk of degrading the mechanical properties of the aluminium is avoided and the legislation is complied with.

[0027] Advantageously, a gas flow is generated in the exchanger cavity during a subsequent step of maintaining the metal inner surface at ambient temperature, during which the solvent in the liquid adhesive is at least partially removed by evaporation, or during a subsequent step of polymerizing the liquid adhesive by heat treatment in a temperature range that does not allow the metal inner surface to interact with the powder solids.

[0028] In the case of solvent evaporation at ambient temperature, a gas flow favors the solvent evaporation. The gas can be preheated to enhance the evaporation. The gas can be, for example, air or nitrogen.

[0029] Advantageously, a subsequent step of maintaining the metal internal surface at ambient temperature with a powder of powdered solid having a particle size between 10 μm and 100 μm, during which the solvent in the liquid adhesive is at least partially removed by evaporation, or a subsequent step of polymerization of the liquid adhesive by heat treatment in a temperature range that does not allow the metal internal surface to interact with the powdered solid, is carried out after the step of removing the powder of the powdered solid that does not adhere to the liquid adhesive deposited on the metal internal surface of the exchanger.

[0030] After this removal, it is easier to proceed to a subsequent step of maintaining the metal interior at ambient temperature, during which the solvent in the liquid adhesive is at least partially removed by evaporation, or to a subsequent step of polymerizing the liquid adhesive by heat treatment in a temperature range that does not allow the metal interior to interact with the powder solids, which would otherwise be hindered by the presence of excess powder, which in fact forms a barrier to solvent evaporation and adds material that is unnecessarily heated in case of polymerization.

[0031] According to one embodiment of the present invention, the liquid adhesive may be polyvinyl alcohol and / or a polymer.

[0032] Advantageously, the liquid adhesive has a viscosity similar to that of water, i.e. 1×10 at 20° C. -3 The liquid adhesive has a viscosity comparable to 10 Pa·s. The liquid adhesive is therefore able to flow into, fill and / or pass through the exchanger cavity, covering all metallic internal surfaces of the exchanger cavity.

[0033] Additionally, the liquid adhesive has sufficient wetting properties on the inner metal surface to leave a layer of adhesive on the inner metal surface after contacting the inner metal surface.

[0034] The wetting properties and viscosity of the liquid adhesive are such that after the liquid adhesive flows into the exchanger cavity, the adhesive remains in the form of a layer of sufficient thickness on all of the metal interior surfaces of the exchanger that have been in contact with the liquid adhesive.

[0035] According to the invention, the thickness of the liquid adhesive that remains attached to the inner metallic surface of the exchanger must be sufficient to adequately adhere the powdered solids during the step of depositing the powdered solids on the deposited liquid adhesive. The thickness of the adhesive layer is, for example, between 5 μm and 50 μm.

[0036] According to the present invention, the particle size of the powder solid should not be too large so that the weight of the particles remains compatible with the adhesive strength of the liquid adhesive. If the particle size is too large, the powder will be too heavy and will not adhere properly to the surface of the exchanger cavity. Therefore, the maximum particle size of the powder solid is selected according to the density of the powder solid, the adhesive strength, and the thickness of the adhesive on the metal inner surface.

[0037] Furthermore, the cross section of the internal cavity is 1 mm 2 ~40mm 2 Since the particle size of the powdered solids is of the order of magnitude of the particle diameter of the powdered solids, the particle size of the powdered solids must remain limited so as not to excessively reduce the cross section of the process fluid flowing through the exchanger cavity during operation of the exchanger.

[0038] Furthermore, a small particle size of the powdered solid is preferred since it increases the exchange surface area of ​​the coating due to the presence of the particles and therefore enhances the action of the powdered solid: taking into account the particle morphology, the active surface area of ​​the powdered solid is much greater than the active surface area of ​​the metallic inner surface of the exchanger which it covers.

[0039] The particle size of the powdered solid is pre-adjusted by grinding and filtration to obtain the appropriate particle size distribution. High energy mills or attritors can be used to obtain small particle sizes.

[0040] According to one embodiment of the invention, the step of depositing liquid adhesive on the inner metal surface is performed by immersing the exchanger in a bath of liquid adhesive or by circulating liquid adhesive through the cavity until the cavity is filled.

[0041] Dipping is a simple method of coating the inner metal surfaces of an exchanger. The exchanger is placed in the bath in an orientation that allows any air in the exchanger cavity to be evacuated and filled with the liquid adhesive. This prevents air from becoming trapped in the heat exchanger and causing the surface to become uncoated. The heat exchanger is kept in the bath for the time required to fill it, for example about 10 minutes.

[0042] Alternatively, the exchanger may be positioned so that its cavity is disposed vertically or tilted with a major vertical component, and liquid adhesive is injected into the exchanger from its top so that liquid flows into the exchanger cavity by gravity flow. The bottom of the exchanger may be blocked so that liquid adhesive can be fed to the exchanger. The bottom of the exchanger may be left free so that excess liquid (that does not adhere to the inside surfaces of the exchanger) can exit the exchanger through its lower end. The amount of adhesive injected must be sufficient to ensure that adhesive is deposited on all the inside surfaces of the exchanger where coating is desired.

[0043] Once the metal interior surface is coated with adhesive, powder solids are deposited.

[0044] In one embodiment of the present invention, the step of depositing the powdered solids on the deposited adhesive is carried out by feeding or pouring. For example, gravity discharge can be used. As with the deposition of the adhesive, the exchanger is then placed with its cavity vertically or tilted with a main vertical component, and a powder of the powdered solids is poured into the exchanger from the top of the exchanger so that the powder of the powdered solids flows into the exchanger cavity. Alternatively, the powder feeding can be carried out by blocking the lower end of the exchanger. The amount of powder poured in such a case must be sufficient to ensure that the powder is deposited on all the metallic internal surfaces of the exchanger where coating is desired.

[0045] According to another embodiment of the invention, the step of depositing powdered solids onto the deposited adhesive is carried out by placing the exchanger in a fluidization chamber in which the powdered solids are pre-suspended using a gas.

[0046] It would be very difficult, if not impossible, to immerse the exchanger in a tank simply containing powdered solids, as the mechanical resistance to the movement of the powder would be too great. By analogy, imagine the difficulty of immersing an exchanger in a sandbox.

[0047] By fluidizing the powder solid, it behaves like a liquid. The heat exchanger can then be immersed in the fluidized bed until it is completely covered. Fluidization also facilitates the flow of the powder into the internal cavity of the exchanger. The exchanger can be placed in an enclosure with its cavity vertically oriented or inclined to the main vertical component to facilitate the flow of the powder through the exchanger.

[0048] By adjusting the descent speed of the exchanger in the fluidization chamber, the cavity is covered with a layer of powdered solid particles with a uniform distribution. Thus, the coverage of the inner metal surface with powdered solids is very high, exceeding the target minimum value of 60%.

[0049] As in the previous approach, the chamber in which the exchanger is immersed can be a temperature-controlled fluidization chamber for adhesive polymerization.

[0050] Advantageously, the gas used to suspend the powdered solids contains an agent which interacts with the powdered solids.

[0051] Advantageously, the agent is designed to initiate or accelerate the polymerization of the adhesive, for example nitrogen hydride to reduce or control the moisture content of the powder solid.

[0052] By using a highly flowable liquid adhesive that completely covers all of the internal metal surfaces of the exchanger, and a small particle powder with a deposition method that can cover all of the previously deposited adhesive, the present invention makes it possible to obtain a consistent coating of powder solids on all of the internal metal surfaces of the exchanger, providing a large surface area for catalytic solids to promote the desired physicochemical reactions.

[0053] According to a second aspect of the invention, a low-temperature heat exchanger is proposed, characterized in that it comprises a metallic internal surface coated with a coating acting as a catalyst for the physicochemical reactions carried out according to the first aspect of the invention.

[0054] Advantageously, the internal metallic surface of the exchanger forms cavities having a length to width ratio of 1000 or more.

[0055] According to one embodiment of the invention, the exchanger is made from aluminum or an aluminum alloy and the powdered solid is an oxide, a hydroxide, a mixture of oxides or hydroxides, or a mixture of oxides and hydroxides.

[0056] Advantageously, the oxide is Fe2O3 or the hydroxide is Fe(OH)3.

[0057] The invention is particularly advantageous for brazed plate and wave type low-temperature exchangers intended for hydrogen production, in which the coating on the metallic inner surface obtained according to the invention acts as a catalyst for the physicochemical reaction of ortho-para hydrogen conversion, which takes place when hydrogen is in the liquid state and at a temperature of about -250°C. [Brief description of the drawings]

[0058] Further features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings. [Figure 1] FIG. 2 is a schematic diagram of a coating method according to a second embodiment of the present invention. [Diagram 2] 2 is a partial schematic diagram of a cold plate heat exchanger according to one embodiment of the present invention after step C of the coating method shown in FIG. 1. [Diagram 3] 3 is a view similar to FIG. 2 after step D of the coating method shown in FIG. 1; [Figure 4] 3 after step F of the coating method shown in FIG. 1; [Diagram 5] FIG. 2 is a schematic view of a portion of an exchanger according to the invention to illustrate the ratio between the width and length of the internal cavity.

[0059] As shown in FIG. 1, in step A, the powder solid, i.e., catalyst, to be deposited on the metal inner surface of the heat exchanger 1 is first sized by mechanical grinding, and then dry filtered to obtain and retain only particles of the appropriate size.

[0060] In step B, the powder obtained is placed in a fluidization chamber where a fluidized bed is formed by circulating gas.

[0061] In parallel, step C prepares the exchanger 1, the inner surface of which forms the substrate or wall to be coated. These surfaces can be etched with a liquid acid solution to ensure perfect adhesion of the adhesive with the powdered solid. Figure 2 shows an enlarged partial cross-sectional view of the exchanger 1 after this step C. The corrugations 2 are arranged between two separating plates 3. The corrugations 2 and the separating plates 3 form a cavity 10 with a metallic inner surface 11.

[0062] Next, in step D, adhesive is coated on the surface of the cavity 10, in this example by dipping. The exchanger 1 is thus immersed in a bath of liquid adhesive at an angle that allows the air inside the exchanger 1 to escape and be replaced by the liquid adhesive. The bath temperature is adjusted to 100° C. for a given adhesive viscosity, e.g. 1×10 at 20° C. -3 The adhesive pressure can be controlled to obtain Pa·s. The exchanger 1 is then removed from the adhesive bath and held above the adhesive bath by placing the exchanger 1 with its cavity oriented vertically. Thus, excess adhesive falls from the exchanger 1 and into the bath. Figure 3 shows a partial cross-sectional view of Figure 2 after step D. The inner surface of the exchanger 1 is covered with adhesive 4.

[0063] Then, in a step E, the exchanger 1 is immersed in the fluidized bed. Advantageously, the descent speed of the exchanger 1 in the fluidized bed is chosen to be between 0.5 mm / s and 1 m / s, more preferably between 1 and 10 mm / s.

[0064] The exchanger 1 is kept in the fluidized bed for a few minutes before being removed.

[0065] Advantageously, the extraction velocity of the exchanger 1 from the fluidized bed is chosen to be between 0.5 mm / s and 1 m / s, more preferably between 1 mm / s and 10 mm / s.

[0066] This is followed in step F by the removal of excess powder, which in this example takes place when the exchanger 1 leaves the fluidized bed. The exchanger 1 is arranged with its cavity 10 oriented vertically above the powder container. Particles that are not held by the adhesive therefore flow out of the exchanger 1 by gravity. To facilitate this flow, the exchanger 1 can be gently shaken and / or gas can be injected into the exchanger 1 to mechanically entrain the loose particles. Figure 4 shows a partial cross-sectional view of Figure 3 after step F. The particles 5 of the powder solid are held on the inner surface of the exchanger 1 by the adhesive.

[0067] It should be noted that the drawings show one exemplary embodiment of the invention in a schematic manner. The ratios between the dimensions of the elements shown do not necessarily represent the actual ratios. Thus, the size of the particles 5 and the thickness of the adhesive 4 in Fig. 4 do not necessarily represent the actual dimensions of the waves and separation plates and the cavities they form, as compared to the dimensions shown.

[0068] In step G, the exchanger 1 is then placed in a temperature controlled chamber where it can be heated to 150° C. for a few tens of minutes to polymerize the adhesive.

[0069] In step H, the exchanger 1 is removed from the temperature chamber and allowed to return to ambient temperature.

[0070] Figure 5 shows a schematic diagram of an exchanger 1 according to the invention. Distance 6 indicates the width of the cavity and distance 7 indicates the length of the cavity. According to the invention, the catalyst coated cavity of the exchanger 1 has a length / width ratio of more than 1000. As shown in the enlarged view 8b of the circular part 8a of the exchanger 1, the corrugations 2 form a cavity 10 with a separation plate 3 of width 6 and height 9. To calculate the ratio between the width and length of the cavity, the width of the cavity is taken to be the smaller of its width or height. In this example, the length / width ratio is 7 / 6.

Claims

1. A method for producing a coating on the metallic internal surfaces forming the cavities of a heat exchanger, said coating intended to act as a catalyst for the physicochemical reaction of ortho-para conversion of hydrogen, said method comprising the following successive steps: - depositing a liquid adhesive onto said metal interior surface, said liquid adhesive being water-based, organic or inorganic; depositing a powder of powdered solids onto the deposited liquid adhesive, wherein the powdered solids are minerals and the powders have particle sizes between 10 μm and 100 μm.

2. 10. The method of claim 1, further comprising, after the powder-depositing step, a subsequent step of maintaining the interior surface at ambient temperature at which the solvent of the liquid adhesive is at least partially removed by evaporation.

3. 2. The method of claim 1, further comprising, after the powder deposition step, a subsequent step of polymerizing the liquid adhesive by heat treatment carried out in a temperature range that does not allow the inner surface to interact with the powder solids.

4. 4. The method of claim 3, wherein the heat treatment is carried out at a temperature of 200°C or less.

5. 4. The method according to claim 2 or 3, characterized in that a gas flow is generated in the exchanger cavity during the subsequent step of maintaining the inner surface at ambient temperature, during which the solvent in the liquid adhesive is at least partially removed by evaporation, or during the subsequent step of polymerizing the liquid adhesive by heat treatment in a temperature range that does not allow the inner surface to interact with the powder solid.

6. 4. The method according to claim 2 or 3, characterized in that the subsequent step of polymerizing the liquid adhesive by heat treatment in a temperature range in which the solvent in the liquid adhesive is at least partially removed by evaporation or the metal inner surface does not allow interaction with the powder solid is carried out after a step of removing the powder of the powder solid deposited on the inner surface of the exchanger that does not adhere to the liquid adhesive.

7. 2. The method of claim 1, wherein the liquid adhesive is polyvinyl alcohol and / or a polymer.

8. The liquid adhesive has a viscosity of water, i.e., 1×10 at 20° C. －３ 8. The method according to claim 1, characterized in that the composition has a viscosity comparable to Pa.s.

9. 10. The method of claim 1, wherein the liquid adhesive has sufficient wettability on the interior surface to leave a layer of adhesive on the interior surface after being contacted therewith.

10. 2. The method of claim 1, wherein the exchanger is made of aluminum or an aluminum alloy and the powdered solid is an oxide or hydroxide, or a mixture of oxides or hydroxides, or a mixture of oxides and hydroxides.

11. The oxide is Fe ２ O ３ or the hydroxide is Fe(OH) ３ The method according to claim 10, characterized in that:

12. 12. The method according to claim 10, wherein the powder of the powdered solid has a particle size of between 50 nm and 1 μm.

13. 2. The method of claim 1, wherein the step of depositing a liquid adhesive on the inner metal surface is performed by immersing the exchanger in a bath of liquid adhesive or by circulating liquid adhesive through the cavity until the cavity is filled.

14. 10. The method of claim 1, wherein the step of depositing the powder of the powder solid onto the deposited adhesive is performed by feeding or pouring.

15. 10. The method of claim 1, wherein the step of depositing the powdered solids onto the deposited adhesive is performed by placing the exchanger in a fluidization chamber in which the powdered solids are pre-suspended with a gas.

16. 16. The method of claim 15, wherein the gas for suspending the powdered solids includes a reagent that interacts with the powdered solids.

17. 17. The method of claim 16, wherein the reagent is intended to initiate or accelerate the polymerization of the adhesive.

18. 2. A low-temperature heat exchanger, characterized in that it comprises a metallic internal surface coated with a coating acting as a catalyst for the physicochemical reactions carried out according to claim 1.

19. 20. The heat exchanger of claim 18, wherein the inner metal surface defines a cavity having a length to width ratio of 1000 or greater.

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